Method and apparatus for providing variable rate data in a communications system using non-orthogonal overflow channels

ABSTRACT

A variable rate transmission system where a packet of variable rate data is transmitted on a traffic channel if the capacity of the traffic channel is greater than or equal to the data rate of the packet. When the rate of the packet of variable rate data exceeds the capacity of the traffic channel, the packet is transmitted on a traffic channel and at least one overflow channel. Also described is a receiving system for receiving and reassembling the data transmitted on the traffic channel and at least one additional overflow channel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/851,655 filed May 8, 2001 now U.S. Pat. No. 7,167,460, which is acontinuation of U.S. patent application Ser. No. 08/937,052, filed Sep.24, 1997, now U.S. Pat. No. 6,292,476, which is a continuation ofapplication Ser. No. 08/838,240, filed Apr. 16, 1997, now U.S. Pat. No.5,777,990, issued Jul. 7, 1998, which is a continuation of U.S. patentapplication Ser. No. 08/395,960, filed Feb. 28, 1995, abandoned(assigned to the same assignee as the present invention), which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to communications. More particularly, thepresent invention relates to a novel and improved communication system,wherein a user transmits variable-rate data on an allocated trafficchannel, but when the user's variable transmission exceeds the capacityof the allocated traffic channel, the user is provided temporary use ofan overflow channel to use with the allocated traffic channel in orderto transmit the high-rate data.

II. Description of the Related Art

The use of code division multiple access (CDMA) modulation techniques isone of several techniques for facilitating communications in which alarge number of system users are present. Other multiple accesscommunication system techniques, such as time division multiple access(TDMA), frequency division multiple access (FDMA) and AM modulationschemes such as amplitude companded single sideband (ACSSB) are known inthe art. However, the spread spectrum modulation technique of CDMA hassignificant advantages over these modulation techniques for multipleaccess communication systems. The use of CDMA techniques in a multipleaccess communication system are disclosed in U.S. Pat. No. 4,901,307,entitled, “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USINGSATELLITE OR TERRESTRIAL REPEATERS,” assigned to the assignee of thepresent invention, of which the disclosure thereof is incorporated byreference. The use of CDMA techniques in a multiple access communicationsystem is further disclosed in U.S. Pat. No. 5,103,459, entitled,“SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULARTELEPHONE SYSTEM,” assigned to the assignee of the present invention, ofwhich the disclosure thereof is incorporated by reference.

The method and apparatus for the generation of a pseudorandom noise (PN)signal that is well suited for CDMA applications is disclosed in U.S.Pat. No. 5,228,054 issued Jul. 13, 1993, entitled, “POWER-OF-TWO LENGTHPSEUDO-NOISE SEQUENCE GENERATOR WITH FAST OFFSET ADJUSTMENT,” assignedto the assignee of the present invention, of which the disclosurethereof is incorporated by reference.

CDMA by its inherent nature of being a wideband signal offers a form offrequency diversity by spreading the signal energy over a widebandwidth. Therefore, frequency selective fading affects only a smallpart of the CDMA signal bandwidth. Space or path diversity is obtainedby providing multiple signal paths through simultaneous links from amobile user through two or more cell-sites. Furthermore, path diversitymay be obtained by exploiting the multipath environment through spreadspectrum processing by allowing a signal arriving with differentpropagation delays to be received and processed separately. Examples ofpath diversity are illustrated in copending U.S. Pat. No. 5,101,501,entitled, “SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM,” andcopending U.S. Pat. No. 5,109,390, entitled, “DIVERSITY RECEIVER IN ACDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of thepresent invention and incorporated by reference herein.

An additional technique that may be used to increase the efficiency ofthe allocation of the communication resource is to allow the users ofthe resource to provide data at varying rates thereby using only theminimum amount of the communication resource to meet their serviceneeds. An example of a variable rate data source is a variable ratevocoder, which is detailed in U.S. Pat. No. 5,414,796, issued May 9,1995, entitled, “VARIABLE RATE VOCODER,” assigned to the assignee of thepresent invention and incorporated herein by reference. Since speechinherently contains periods of silence, i.e. pauses, the amount of datarequired to represent these periods can be reduced. Variable ratevocoding most effectively exploits this fact by reducing the data ratefor these periods of silence.

In a variable rate vocoder of the type described in aforementioned U.S.Pat. No. 5,414,796, approximately 40% of the speech packets are coded atfull rate. In the vocoder described in the patent application, theencoding rate is selected in accordance with the packet energy. When thepacket energy exceeds a full rate threshold the speech is coded at fullrate. In U.S. patent application Ser. No. 08/288,413, entitled,“IMPROVED METHOD AND APPARATUS FOR SELECTING AN ENCODING RATE IN AVARIABLE RATE VOCODER,” assigned to the assignee of the presentinvention and incorporated herein by reference, a method for determiningbased on characteristics of the speech packet if some of the packets tobe coded at full rate can be coded at a lower rate without sacrificingperceived quality.

A variable rate speech encoder provides speech data at full rate whenthe talker is actively speaking, thus using the full capacity of thetransmission packets. When a variable rate speech coder is providingspeech data at a less that maximum rate, there is excess capacity in thetransmission packets. A method for transmitting additional data intransmission packets of a fixed, predetermined size, wherein the sourceof the data for the data packets is providing the data at a variablerate is described in detail in copending U.S. Pat. No. 5,504,773, issuedApr. 2, 1996, entitled “METHOD AND APPARATUS FOR THE FORMATTING OF DATAFOR TRANSMISSION,” assigned to the assignee of the present invention, ofwhich the disclosure thereof is incorporated by reference herein. In theabove-mentioned patent application a method and apparatus is disclosedfor combining data of differing types from different sources in a datapacket for transmission.

SUMMARY OF THE INVENTION

A communications resource is typically divided into communicationschannels. Typically, each of these channels has the same capacity. Acommunications system could re-allocate the channels to the users foreach transmission. This would theoretically allow for a maximallyefficient allocation of the communication resource because each userwould be using only the amount of the resource absolutely necessary.However, this technique would result in unacceptable complexity in theresulting receiver and transmitter design.

In the present invention, an efficient method of transmitting andreceiving variable rate data is disclosed. In the present invention,each user is provided with a voice or data channel, also referred to asa traffic channel, specifically allocated for that user. In addition,each user is provided with selective access to a pool of overflowchannels that are for use by all users of the communications resource.If a user needs to transmit at a rate higher than the capacity of theallocated traffic channel then the user transmits the information usingboth the allocated traffic channel and an overflow channel.

In the exemplary embodiment, the communication system is a code divisionmultiple access (CDMA) communication system as is described in detail inthe aforementioned U.S. Pat. Nos. 4,901,307 and 5,103,459. In theexemplary embodiment each of the traffic channels are orthogonal to oneanother. Each traffic channel is spread by a unique Walsh sequence thatis orthogonal to the other Walsh sequences. The spread signals are thenspread by pseudorandom noise (PN) sequences and then transmitted.

In the exemplary embodiment, the overflow channels are not provided withunique orthogonal Walsh spreading sequences, because this would decreasesystem capacity. Instead, the system spreads the overflow channelportion of the information by a Walsh sequence that is not unique fromthose used in spreading the traffic channels. This portion is thenspread by a PN sequence. The PN sequence is unique from the PN sequenceused to spread the traffic channel of the same Walsh sequence. In theexemplary embodiment the traffic channel and the overflow channel use,although not necessarily, the same Walsh spreading sequence.

In the exemplary embodiment, the receiver continuously monitors both thetraffic channel and the overflow channel. If the receiver determinesthat information is being transmitted on both the traffic channel and anoverflow channel then the receiver decodes both portions of the message,combines the portions and provides the decoded message to the user. Inan alternative embodiment, the receiver need not continuously monitorthe overflow channel, but rather only monitors the overflow channel wheninstructed by information on the traffic channel that directs thereceiver to monitor the overflow channel.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout:

FIG. 1 is a diagram illustrating an exemplary implementation of thepresent invention in a satellite communication system;

FIGS. 2 a-2 d are an illustration of exemplary transmission packetstructures of the exemplary embodiment;

FIGS. 3 a-3 e are an illustration of the symbol repetition in atransmission packet and the transmission energy level of the packet;

FIG. 4 is a block diagram of the transmission system of the presentinvention; and

FIG. 5 is a block diagram of a receiver system of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A multiple access communication resource is divided into parcelsreferred to as channels. This division is called multiplexing. Threespecific types of multiplexing are frequency division multiplexing(FDM), time division multiplexing (TDM), and code division multiplexing(CDM). The basic unit of information transmitted and received in acommunication system is referred to as a packet.

Referring now to the figures, FIG. 1 illustrates the exemplaryimplementation of the present invention. In FIG. 1, the presentinvention is implemented in a satellite communications network. Itshould be understood that the present invention may be utilized in aterrestrial-based system, such as one where base stations are used tocommunicate with remote stations. The network is used to communicateinformation to a remote user station or terminal 2 from gateway 8 viasatellites 4 and 6, which may be either geosynchronous or low earthorbit (LEO) types. User terminal 2 may be a mobile station, such as aportable telephone or other portable or mobile communications device, oruser terminal 2 may be a fixed communications device, such as a wirelesslocal loop terminal or a central communications center, such as acellular base station. Although only two satellites, a single userterminal, and a single gateway are shown in FIG. 1 for ease inillustration, a typical system may contain a plurality of all.

In the exemplary embodiment, satellites 4 and 6 are transponders ornon-regenerative repeaters that simply amplify and re-transmit thesignal received from gateway 8. The present invention is equallyapplicable to cases where satellites 4 and 6 are regenerative repeatersthat demodulate and reconstitute the signal prior to re-transmission. Inthe exemplary embodiment, the signal transmitted from gateway 8 tosatellites 4 and 6 is a spread spectrum signal. In addition, the signalstransmitted from satellites 4 and 6 to user terminal 2 are spreadspectrum communications signals. The generation of spread spectrumcommunications signals is described in detail in the aforementioned U.S.Pat. Nos. 4,901,307 and 5,103,459.

Gateway 8 serves as an interface from a communication network to thesatellites 4 and 6, or directly to terrestrial base stations (aconfiguration not shown). Gateway 8 is typically a centralcommunications center that receives data via a network (not shown),which may include public switching telephone networks (PSTN) andnetworks specifically designed for the communications of the presentinvention. Gateway 8 may be connected to the network (not shown) bywireline communications or by means of an air interface. Gateway 8provides the data received from the network via satellites 6 and 8 touser terminal 2. Similarly, gateway 8 provides data received from userterminal 2 via satellites 4 and 6 to the network.

In the exemplary embodiment, the communications network transmitsvariable rate data from gateway 8 to user terminal 2. A variable ratecommunication system communicates data where the rate of the datacommunicated varies with time. In the exemplary embodiment, thecommunications resource is divided into channels. In the exemplaryembodiment, each channel has the same capacity.

In the exemplary embodiment gateway 8 communicates to user terminal 2 atone of four different information data rates. These data rates, orderedfrom lowest rate to highest rate, are referred to, as eighth rate,quarter rate, half rate and full rate. In the exemplary embodiment, asingle traffic channel has adequate capacity to carry the packet of allrates except full rate, which requires a traffic and an overflowchannel. In the exemplary embodiment, a traffic channel can carrypackets of 96 bits or less. Half rate, quarter rate and eighth ratepackets consist of 96, 48 and 24 bits, respectively. A full rate packetconsists of 192 bits and so requires a traffic channel plus an overflowchannel of equal capacity.

The present invention is easily extendible to cases where there are moreor less than four rates, where the channels can carry data at a lessersubset of the possible rates, or where the highest rate requires morethan two channels. The communications system of the present inventioncan communicate both fixed rate data and variable rate data. In thecommunication of fixed rate data a channel or set of channels is to beallocated for the duration of the service being provided.

In the exemplary embodiment, the channels are designated into twogroups. A first group of channels is the traffic channel group. Usersare allocated traffic channels or sets of traffic channels specificallyfor their use for the duration of service. In the exemplary embodiment,all traffic channels are orthogonal to one another. In the exemplaryembodiment this orthogonality is attained by allocating a unique Walshsequence to each user, as is described in detail in the aforementionedU.S. Pat. Nos. 4,901,307 and 5,103,459. The information packets arespread, i.e., combined, with an orthogonal function sequence, typically,a Walsh sequence, then the Walsh spread packet is mixed, i.e., spreadspectrum processed with a pseudorandom noise (PN) sequence. Furtherdetails on the spread spectrum modulation of the information packets areprovided in the aforementioned U.S. Pat. Nos. 4,901,307 and 5,103,459.

Overflow channels are not provided with unique orthogonal or Walshsequences and, as such, are not assured of being orthogonal to alltraffic channels. However, the PN sequence which is mixed with thespread packet is unique, so all other packets will appear as noise tothe decoder of the overflow channel and the overflow channel informationcan be distinguished from all traffic channel information.

Table 1 below illustrates the numerology used in the exemplaryembodiment of the present invention.

TABLE 1 EXEMPLARY NUMEROLOGY OF THE PRESENT INVENTION Parameter UnitsData Rate 8600 4000 1700 800 bps PN Chip 1.2288 1.2288 1.2288 1.2288Mcps Rate Code Rate 1/2 1/2 1/2 1/2 bits/code symbol Code 1 1 2 4 modsym/ Repetition code sym No. of 2 1 1 1 Channels Modulation BPSK BPSKBPSK BPSKThe present invention is equally applicable to other numerologies.

FIG. 4 illustrates the transmission system of the present invention.Input data for transmission is provided to variable rate data source 50.Variable rate data source 50 provides the variable rate data to encoder52. In the exemplary embodiment, variable rate data source 50 providesdata at four different rates referred to as full rate, half rate,quarter rate and eighth rate. In the exemplary embodiment, full rate is8.6 kbps and provides packets of 172 bits, half rate is 4 kbps andprovides packets of 80 bits, quarter rate is 1.7 kbps and providespackets of 34 bits, and eighth rate is 800 bps and provides packets of16 bits.

The exemplary embodiment of variable rate data source 50 is a variablerate vocoder as described in the aforementioned U.S. Pat. No. 5,414,796.In the exemplary variable rate vocoder, the energy of a packet of speechdata is measured and compared to a set of threshold values, whichdetermine the encoding rate. The aforementioned U.S. patent applicationSer. No. 08/288,413 teaches of methods that minimize the number ofpackets encoded at full rate with minimum impact on perceptual quality.

Variable rate data source 50 encodes the input data and provides it atone of the predetermined rates. In the exemplary embodiment, a trafficchannel is capable of carrying packets encoded at or below half rate.When a packet of data is encoded by variable rate data source 50 at fullrate, then the packet must be transmitted using both a traffic channeland an overflow channel.

The data packet provided by variable rate data source 50 is provided toencoder 52. In the exemplary embodiment, encoder 52 generates a set ofredundant bits in accordance with error correction and detection methodsthat are well known in the art. In the exemplary embodiment, theredundant bits are cyclic redundancy check (CRC) bits, the generation ofwhich is well known in the art, and is detailed in the aforementionedU.S. Pat. No. 5,504,773.

FIGS. 2 a-d illustrate the packet structures of the exemplaryembodiment. FIG. 2 a illustrates the packet structure of a full ratepacket consisting of 172 information bits followed by 12 redundant bitsand then by 8 tail bits. FIG. 2 b illustrates the packet structure of ahalf rate packet consisting of 80 information bits followed by 8redundant bits and then by 8 tail bits. FIG. 2 c illustrates the packetstructure of a quarter rate packet consisting of 34 information bitsfollowed by 6 redundant bits and then by 8 tail bits. FIG. 2 dillustrates the packet structure of an eighth rate packet consisting of16 information bits followed by 8 tail bits.

Referring back to FIG. 4, encoder 52 then encodes the formatted packetfor error detection and correction as is well known in the art. In theexemplary embodiment, encoder 52 encodes the formatted packet inaccordance with a rate 1/2 convolutional code. Encoder 52 provides theencoded packet to interleaver 54.

Interleaver 54 interleaves the binary symbols of the encoded packet inaccordance with a predetermined interleaver format. In the exemplaryembodiment, interleaver 54 is a block interleaver. In addition,interleaver 54 provides redundancy in the packets such that eachinterleaved packet consists of the same number of binary symbols asdescribed below.

Referring to FIGS. 3 a-3 e in conjunction with FIG. 4, interleaver 54interleaves the binary symbols of the packet, then groups the reorderedbinary symbols into power control groups. FIGS. 3 a and 3 b illustrate afull rate packet organized into a packet format. Because a full ratepacket of a transmission packet requires two channels, the first portionof the packet illustrated in FIG. 3 a is organized into a traffic packetand transmitted on the traffic channel. The second portion of the fullrate packet as illustrated in FIG. 3 b is organized into an overflowpacket and transmitted on an overflow channel. For the full rate packetinterleaver 54 does not provide symbol repetition. Since the symbol datafills the traffic channel and the overflow channel packet. In theexemplary embodiment, each power control group consists of 12 binarysymbols. FIG. 3 c illustrates a half rate packet organized into a packetformat. Note that because transmission of the half rate packet utilizesthe full capacity of the traffic channel packet, there is no symbolrepetition provided in the packet. FIG. 3 d illustrates a quarter ratepacket organized into a packet format, in which each symbol is providedtwice. FIG. 3 e illustrates an eighth rate packet organized into packetformat, in which each symbol is provided four times.

Referring again to FIG. 4, the interleaved packet is provided byinterleaver 54 to de-multiplexer 56, which operates in accordance with arate signal provided by variable rate data source 50. If the packet issuch that it can be carried on the traffic channel without need of anoverflow channel, then the interleaved packet is provided throughde-multiplexer (DE-MUX) 56 without any change to modulator 57. If, onthe other hand, the packet requires use of an overflow channel fortransmission, then de-multiplexer 56 splits the packet into twoportions. The de-multiplexed packet is provided by de-multiplexer 56 tomodulator 57. Buffering may be added to ensure two portions aresimultaneously provided to modulator 57.

In the exemplary embodiment, if the packet is of a rate less than fullrate, then the entire packet is provided to combining element 58, whichin the exemplary embodiment is a digital multiplier modulo 2 adder orexclusive or gate. The interleaved packet is spread by an orthogonalfunction W_(i), as is described in detail in the aforementioned U.S.Pat. No. 5,103,459. In the exemplary embodiment, orthogonal functionW_(i) is a Walsh function the selection of which is detailed in theaforementioned U.S. Pat. No. 5,103,459. In the exemplary embodiment,each Walsh sequence, W_(i), is uniquely provided for use by User (i).

The orthogonal function spread packet from element 58 is provided toquadrature spreading elements 62 and 64 which are employed as digitalmultipliers modulo 2 adder or exclusive or gates. The orthogonalfunction spread packet is spread in quadrature spreading elements 62 and64 by pseudorandom noise (PN) functions PN_(I) and PN_(Q), respectively.PN_(I) and PN_(Q) are generated by traffic PN generator 63. In theexemplary embodiment, the pseudorandom noise signals are generated by amaximal shift register where the PN sequence is determined by thetemporal offset of the register. The design and implementation of suchregisters is described in detail in the aforementioned copending U.S.Pat. No. 5,228,054. The quadrature spread packets are provided fromquadrature spreading elements 62 and 64 to transmitter 72.

Transmitter 72 converts the signal to analog form, frequency up-convertsand amplifies the signal for transmission transmitter 72 amplifies thesignal in accordance with the rate of the packet. The relationshipbetween the necessary transmission energy and the amount of repetitionis illustrated in FIGS. 3 a-3 e. When a redundancy is present in apacket, the packet can be transmitted at a lower energy with redundantportions combined at the receiver. In a full rate packet, both thepacket on the traffic channel (traffic packet) FIG. 3 a and the packettransmitted on the overflow channel (overflow packet) FIG. 3 b aretransmitted at a maximum bit energy E. In FIG. 3 c, there is norepetition in the half rate packet so the packet is also provided atenergy level E. In FIG. 3 d, there is a repetition rate of two so thepacket is provided at half the packet energy of the half rate packet orE/2. In FIG. 3 e, there is a repetition rate of four so the packet isprovided at a quarter of the packet energy of the half rate packet orE/4. Transmitter 72 amplifies and up-converts the signal and provides itto antenna 74 for broadcast to a receiver.

In the case where a packet requires use of an overflow channel fortransmission that is the packet is a full rate packet, thende-multiplexer 56 splits the interleaved packet into two halves andprovides a first half to combining element 58 and a second half of thepacket to combining element 60, which is also embodied as a digitalmultiplier, modulo 2 adder or an exclusive-or gate. The modulation ofthe first half of the packet proceeds as described previously forpackets of less than full rate. The modulation of the second half of thepacket is the same except that the spreading functions are different.

Now turning to the specifics of the modulation process of modulator 57,it should be noted that a spread spectrum communication system isprimarily limited in capacity by the number of unique orthogonalfunctions or in the exemplary embodiment Walsh functions available. Ifone allocates a subset of these functions for the purpose of modulatingthe overflow channels, then the capacity of the system is reduced.

In the present invention, all of the traffic channels are orthogonal toone another because each is modulated by a unique Walsh function (W_(i))or Walsh code sequence. However, the Walsh functions provided formodulation of the overflow channel (W_(j)) will overlap those allocatedfor modulation of the traffic channel and so are not orthogonal to thetraffic channels. In the exemplary embodiment, W_(i) is the same Walshfunction as W_(j). That is, the same Walsh code function spreads thetraffic channel portion of the packet as it spreads the overflow portionof the packet. The two signals are distinguished from one another bysubsequent PN code sequence spreading.

The spread packet from combining element 60 is provided to quadraturespreading elements 66 and 68, which may be embodied as digitalmultipliers, modulo 2 adders, or exclusive-or gates. Quadraturespreading elements 66 and 68 spread the packet from spreading element 60in accordance with pseudorandom noise functions PN_(I′) and PN_(Q′).Pseudorandom noise functions PN_(I′) and PN_(Q′) are generated byoverflow PN generator 67. In the exemplary embodiment, pseudorandomnoise functions PN_(I′) and PN_(Q′) are generated in the same manner aspseudorandom noise functions PN_(I) and PN_(Q), but are unique due todifferent temporal offsets in the code sequences.

In a first exemplary embodiment, pseudorandom noise functions PN_(I) andPN_(Q) and pseudorandom noise functions PN_(I′) and PN_(Q′) are shortcodes. A short code is a code in which the number of sequences generatedis relatively few. In the exemplary embodiment, the PN generatorprovides a bit stream that is periodic with a period of 2¹⁵ PN chips.The benefit of this implementation is that it allows a fasteracquisition at the receiver. In an alternative embodiment, pseudorandomnoise functions PN_(I) and PN_(Q) are short codes, but pseudorandomnoise functions PN_(I′) and PN_(Q′) are long codes. This has thepotential of advantage of greater separation in code space, whileallowing the mobile to acquire using the demodulation of the firstsubpacket modulated by the short code.

The modulated overflow packet from quadrature spreading elements 66 and68, as well as the modulated traffic packet from quadrature spreadingelements 62 and 64 are provide to transmitter 72. Transmitterup-converts and amplifies the signal and provides the signal to antenna74 for broadcast. The subpackets are transmitted with energy E asillustrated in FIGS. 3 a and 3 b (vertical axis).

FIG. 5 illustrates the receiver system of the present invention. Thetransmitted signal is received at antenna 100 and provided to receiver102. Receiver 102 down converts and amplifies the signal and providesthe signal in its two components I and Q to overflow channel despreadingelement (OVERFLOW DESPREADER) 104 and to traffic channel despreadingelement (TRAFFIC DESPREADER) 120. In the exemplary embodiment, the Icomponent and the Q component of the received signal carry the samedata, which allows for a higher quality reception. In an alternativeembodiment, the I and Q components could carry different data whichwould allow for higher data rate. In the exemplary embodiments,despreaders 104 and 120 are configured as QPSK despreading circuits asare well known in the art.

The received signal is provided to traffic channel despreading element(TRAFFIC DESPREADER) 120, which despreads the signal in accordance withthe traffic channel pseudorandom noise codes PN_(I) and PN_(Q). TrafficPN generator 119 generates the PN_(I) and PN_(Q) sequences. In theexemplary embodiment, the PN sequences are generated by a shift registerwith appropriate feedback. The despreading process involves digitallymultiplying the signal by the traffic channel pseudorandom noise codesPN_(I) and PN_(Q). This process is described in detail in theaforementioned U.S. Pat. Nos. 4,901,307 and 5,103,459. Similarly, thereceived signal is provided to overflow channel despreading element(OVERFLOW DESPREADER) 104, which despreads the signal in accordance withthe overflow channel pseudorandom noise codes PN_(I′) and PN_(Q′).Overflow PN generator 103 generates the PN_(I′) and PN_(Q′) sequences.In the exemplary embodiment, the PN sequences are generated by a shiftregister with appropriate feedback. In the exemplary embodiment, the twogenerators are identical except that they are offset from one anothertemporally, which means that the traffic PN sequences and the overflowPN sequences will be unique from one another.

The despread signal is then provided to demodulation elements 105 and121. Demodulation element 121 receives the traffic channel despread Iand Q values and demodulates the signal. The demodulation process isillustrated by the digital multiplication of the signal by the Walshfunction W_(i) in multipliers 122 and 124 then the accumulation of themultiplied signal in accumulators 126 and 128. Similarly, thedemodulation of the overflow channel is illustrated by the digitalmultiplication by the Walsh function W_(j) in multipliers 106 and 110,then the accumulation of the multiplied signal in accumulators 108 and112.

The demodulated signal is provided by traffic demodulator 121 tocombiner element 130 and by overflow demodulator 105 to combiner element114. The combiner elements combine received and despread data estimatesfrom despreaders 105 and 121 with data estimates from otherreceiver/despreader/demodulators demodulated fingers (not shown) thatare simultaneously being tracked by the receiver system. These otherestimates take advantage of the delayed signals resulting from multipathsignals to provide an improved signal estimate. The design andimplementation of combiner elements are described in detail in theaforementioned U.S. Pat. Nos. 5,101,501 and 5,109,390.

The combiners combine the signals based upon the values of the data andthe relative strengths of the signals and provide combined estimates tode-interleaving element (DE-INT) 116. De-interleaving element 116re-orders the combined estimates of the data in accordance with apredetermined ordering format and provides the reordered data to decoder(DECODE) 118. Decoder 118 decodes the data in accordance with apredetermined decoding format. In the exemplary embodiment, decoder 118is a Viterbi decoder of constraint length 7. The decoded packet is thenprovided to the receiver system user.

In an improved embodiment of the communication system of the presentinvention, an alternative modulation and demodulation process isprovided for when system usage is low. When system usage is low, eachuser is provided use of one of the unique Walsh sequences forcommunication of its overflow data. That is, W_(i) and W_(j) aredifferent so the traffic and overflow signals are orthogonal to oneanother. In the exemplary embodiment, W_(i) and W_(j) are separated by afixed offset from one another so that the receiver knows which Walshsequence to use to demodulate the overflow signal. In the exemplaryembodiment of a communication system of 128 unique Walsh sequences, whenusage is low, each user is allocated a traffic channel designated byWalsh sequence W_(i) and uses an overflow channel designated by Walshsequence W_(j)=W_(i)+64.

When system usage rises so that the system can no longer accommodatethis many unique overflow channels, that is 65 or more users, then thesystem transmitter will send signaling information to the receiverindicating that the overflow communications will be conducted asdescribed previously, using the same Walsh sequence for both traffic andoverflow communications. The users may be switched to the high usagemode individually as necessary or as a group.

The previous description of the preferred embodiments is provided toenable any person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

1. A system for receiving variable rate packets of data symbolscomprising: a traffic demodulator using a first plurality of sequencesfor demodulating a first half of the variable rate packets, the trafficdemodulator having an input and having an output, wherein the firstplurality of sequences is orthogonal; an overflow demodulator using asecond plurality of sequences for demodulating a second half of thevariable rate packets, the overflow demodulator having an input andhaving an output, wherein the second plurality of sequences isnon-orthogonal to the first plurality of sequences; and a combinerhaving a first input coupled to said traffic demodulator output andhaving a second input coupled to said overflow demodulator output andhaving an output for producing estimates of ones of said variable ratepackets received by said system which include in excess of a predefinednumber of data symbols.
 2. The system of claim 1, wherein the firstplurality of sequences is a Walsh code sequence.
 3. The system of claim1 wherein at least one of the variable rate packets is a full ratepacket.
 4. The system of claim 3 wherein at least one other of thevariable rate packets is either an eighth rate packet, a quarter ratepacket or a half rate packet.
 5. The system of claim 3 furthercomprising a traffic despreader for despreading the input of the trafficdemodulator.
 6. The system of claim 5 further comprising an overflowdespreader for despreading the input of the overflow demodulator.
 7. Thesystem of claim 6 wherein the traffic despreader use a first PN sequencefor despreading and the overflow despreader uses a second PN sequencefor despreading.
 8. The system of claim 7 wherein the first PN sequenceis unique from the second PN sequence.